1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a scheduler and a scheduling method that may select a data transmission time interval and a data transmitting station and/or data transmitting antenna in a multi-user Multiple Input Multiple Output (MIMO) communication system.
2. Discussion of the Background
A wireless local area network (WLAN) is a local area network where at least some nodes of the network are connected without using a cable. The WLAN may avoid the cost of setting up a cable-based network, and may also provide users with the convenience of a network access environment, while maintaining the simple implementation and expandability of a wired LAN.
Various types of portable devices, such as a portable digital assistant (PDA), a portable media player (PMP), a tablet PC, and the like, are currently being used. Also, many users desire an ability to connect to a network with the portable devices. Therefore, there is an increasing interest regarding the WLAN.
A representative standard associated with the WLAN may include an Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless-Fidelity (Wi-Fi) standard. Since the IEEE 802.11 standard was released in 1997, revisions including 11a, 11b, and 11g have been implemented. The IEEE 802.11n version is currently in development.
The IEEE 802.11n version, which is still in the standardization process, may adopt a system configuration based on a MIMO communication scheme to support a higher data transmission rate in a physical layer. The MIMO communication scheme is a scheme where a transmission end may transmit data via multiple paths using multiple transmit antennas, and a reception end may receive data via multiple paths using multiple receive antennas. Through this, the MIMO communication scheme may enhance a data transmission rate and may reduce interference in a multi-path environment.
Accordingly, in the IEEE 802.11n WLAN, a station and an access point each may include multiple antennas. Through this configuration, the IEEE 802.11n WLAN may support an enhanced data transmission rate in a physical layer in comparison to existing versions. However, even if performance is enhanced in the physical layer, there may be some constraints on improving the data transmission throughput due to a limit of a protocol of a Media Access Control (MAC) layer.
Stations accessing a particular access point may transmit data to the particular access point in the same time slot. If the particular access point includes only a single receive antenna, the access point may not successfully receive all data transmitted from the stations. Specifically, if two stations simultaneously transmit data streams, the access point having a single receive antenna may not successfully receive the data streams. Instead, the simultaneously transmitted data streams may collide with each other and an error may occur in the data streams.
In this case, each station may not receive an acknowledgement (ACK) from the access point and thus may retransmit the data.
If a single access point covers a small number of stations, a collision probability may be lower so that performance deterioration according to frame retransmission may be insignificant. However, as a number of stations increases, the collision probability may also increase and thus a data transmission throughput may deteriorate.
Accordingly, there is a need for a new technology that may reduce the risk of data collision by applying a MIMO technology to a scheduler of a MAC layer.